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Regulatory Assets and Liabilities (Regulated Assets) (Details) - CAD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Current | $ 165 | $ 138 |  |  |
Regulatory Assets, Noncurrent | 1,404 | 1,273 |  |  |
Regulatory Assets, Total | 1,569 | 1,411 |  |  |
Deferred income tax regulatory assets [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 775 | 667 |  |  |
Pension and post-retirement medical plan [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 453 | 380 | [1] |  |
Pension and post-retirement medical plan [Member] | Prior period reclassification [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total |  | 35 |  |  |
Cost Recovery Clauses [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 75 | 17 |  |  |
Environmental remediations [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 31 | 41 |  |  |
GBPC Hurricane Matthew restoration [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 28 | 28 |  |  |
Stranded cost recovery [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 28 | 25 |  |  |
Unamortized Defeasance Costs [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 26 | 32 |  |  |
Demand Side Management [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 24 | 28 |  |  |
Demand Side Management [Member] | Nova Scotia Utility and Review Board [Member] | NSPI [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
DSM deferral |  |  |  |  | $ 35
Demand Side Management [Member] | EfficiencyOne [Member] | NSPI [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
DSM deferral |  |  |  | $ 31 |
Deferrals related to derivative instruments [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 10 | 15 |  |  |
Storm Reserve [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | 4 | 59 |  |  |
Other Regulatory Assets (Liabilities) [Member] |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |
Regulatory Assets, Total | $ 115 | $ 119 |  |  |
[1] | The December 31, 2017 pension and post-retirement medical plan regulatory asset includes a prior period reclassification of $35 million from AOCI, for changes in unrecognized pension and post-retirement benefit costs to be consistent with current year presentation. Refer to note 10 for further details.

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- Definition Carrying amount as of the balance sheet date of capitalized costs of a regulated entity, consisting of the planning, implementing, and monitoring activities which are designed to encourage consumers to modify their level and pattern of energy usage, which are expected to be recoverable through rate adjustments after one year or beyond the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77907324&loc=d3e43603-110378 + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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